Parasites may play a complex role in shaping patterns of host reproduc
tive effort and therefore in determining costs of reproduction in thei
r hosts. Parasites may cause a more or less large reduction in the amo
unt of energy an individual host may use for reproduction and maintena
nce. Irrespective of the parasite-induced reduction in energy intake,
infected hosts may either show the same allocation pattern than uninfe
cted hosts, or respond to parasitism by changing the proportion of ene
rgy allocated to each trait (e.g. increased investment in current repr
oduction at the expense of survival). Moreover, hosts may differ in th
eir susceptibility to parasites (e.g. hosts with high reproductive eff
ort mag show higher vulnerability to parasite infection). Each of thes
e hypotheses leads to a pattern of correlation between parasite load,
host survival and reproduction. In this paper we compared the predicte
d patterns of correlation between parasite load, host survival and rep
roduction with those observed for the Lacerta vivipara - haematozoa as
sociation. We found that: (1) haematozoa load was positively correlate
d with lizard reproductive effort; (2) haematozoa load was not correla
ted with host survival; (3) lizards with high reproductive investment
suffered higher mortality rare than lizards with low reproductive inve
stment. These findings suggest that parasites do not significantly red
uce host energy intake, and are in agreement with a shift of the alloc
ation rule of parasitized hosts toward reproduction. However, our corr
elative study does not allow us to attribute this shift to an adaptive
host response to parasites rather than a by-product of a differential
susceptibility to parasite infections.